In the manufacture of monovinyl polymers such as styrene homopolymer, elastomer modified styrene homopolymer, styrene copolymers with acrylonitrile, methacrylic acid esters, and maleic acid derivatives, both with and without elastomeric modification and acrylic polymers, by a continuous polymerization method, it is necessary to remove a fraction of volatile components such as un-reacted monomers, solvents, and impurities from a viscous polymer liquid composed of these volatile components and the polymer product. To accomplish such removal, producers of the named polymers widely practice a step in the manufacturing process known in the industry as devolatilization.
Devolatilization is required for polymer products entering the general consumer market in order to eliminate odors from the small quantities of volatile components that remain in the polymer after production, and to meet the technical requirements of polymers that are used for food-contact applications.
Devolatilization is generally completed in one or more stages involving heating and exposing the viscous polymer solution stream to elevated temperatures and to reduced pressure, or vacuum, where the volatile components are vaporized and removed from the purified polymer. The degree of removal of the volatile components from the polymer product is typically greatly restricted by a combination of the high viscosity of the liquid polymer stream, and the general desire to reduce the duration of polymer exposure to the elevated temperatures that are required for effective devolatilization. These restrictions motivate producers of such polymers to employ a variety of devices to improve the distribution and exposure of viscous polymer streams to vacuum, thereby obtaining a final polymer product with minimum levels of volatile impurities.
The distribution and exposure of the viscous polymer liquid to vacuum is improved with two general types of liquid distribution devices. The first such distributor device relies on pressure generated in upstream equipment to force the viscous polymer liquid stream through a manifold and thence through a plurality of flow channels which may take a variety of shapes, the intent of which is to maximize the surface area of viscous liquid exposed to reduced pressure, and will be referred to herein as a “pressurized distributor.” The second such distributor device relies on gravity to distribute the viscous stream over and/or through the distributor to form one or more streams or strands. This second device may be an open pipe discharge; or discharge over one or more flat or tilted plates, shaped plates, sieve plates or assemblies, slotted plates or assemblies; or a combination of the previously named plates or assemblies with weirs that serve to add residence time. These types of distributors are referred to herein as “gravity distributors.”
Some manufacturers use only one heating step for all stages, while others use a heating step before each distribution and vacuum exposition stage. Additionally, the incorporation of a small fraction of a highly volatile stripping agent such as steam or methanol between stages is sometimes practiced.
Gordon et al., in U.S. Pat. No. 3,853,672 (Dec. 10, 1974), disclose an apparatus for an improved falling strand devolatilizer. The devolatilizer includes a shell and tube heat exchanger whose tubes discharge as falling strands into a first vessel operating under a level of vacuum provided by a gas pump attached to the first vessel. The first vessel is connected to a second, independent vessel that operates at a higher degree of vacuum, via an actuated valve that controls the flow from and level in the first vessel. Each vessel is designed and constructed as a separate vessel. The claims include a liquid pump to empty the second vessel, a level sensor in the first vessel, and a level controller. The claims are also limited to vessels with generally tapering lower regions terminating at a discharge port.
Hagberg, in U.S. Pat. No. 3,928,300 (Dec. 23, 1975), discloses a process for devolatilizing polystyrene in essentially the same device disclosed by the Gordon patent above. Hagberg claims a process for devolatilizing styrene homopolymer that minimizes the oligomer content in said styrene homopolymer by exposing the tubes of the shell and tube heat exchanger to various levels of vacuum in the first vessel, and passing the polymer solution by gravity and differential pressure to the second vessel that operates at a fixed, higher level of vacuum. Hagberg shows a reduction of styrene oligomer content in the product from 1.7% to 1.2% by adjusting the first vessel pressure from 760 to 50 mm Hg absolute.
Hagberg, in U.S. Pat. No. 3,966,538 (Jan. 29, 1976), discloses the apparatus for the Hagberg patent above (which is essentially the same as the Gordon patent), differing only in modifying the method of attachment of the heat exchanger to insert the discharge tubesheet into the first vessel. The patent also claims the embodiment of this heat exchanger and vessel combination where the second vacuum vessel is not used.
These three aforementioned patents have in common a design whereby the viscous liquid enters the first vacuum vessel through a heat exchanger whose tubes discharge the liquid directly into the first vessel as partially devolatilized falling strands. Further, the liquid passes from the first vessel by gravity and differential pressure through a valve into a second vacuum vessel, which is maintained at a higher vacuum relative to the first vessel.
Newman, in U.S. Pat. No. 4,294,652 (Oct. 13, 1981), discloses an improvement to the apparatus of the above-described Gordon and Hagberg patents. In particular, the second vacuum vessel is partitioned into two compartments with a means of circulating viscous liquid from one side to the other. A baffle is used to divert the flow to one compartment, and a weir is used to separate the tank bottom into two compartments of substantially equal size. The circulated material may be transferred through an orifice to increase the surface area of the falling viscous liquid during devolatilization. The main objective of this patent is to modify the apparatus and process of Gordon and Hagberg to allow the incorporation of steam as a stripping agent to the second vacuum vessel.
McCurdy et al., in U.S. Pat. No. 4,439,601 (Mar. 27, 1984), disclose a multistage devolatilization apparatus that comprises a heater followed by two vacuum vessels operating at less than atmospheric pressure. The second vessel operates at a pressure below the first vessel. The vaporized volatile components that are removed from the second vacuum vessel are recombined with the vaporized volatile components from the first vacuum vessel. The arrangement of the heater and the first vacuum vessel is not specified. The method in which the material passes from the first vessel into the second vessel is not specified, and neither is the means for allowing the recombining of vapor from the first and second vessels. This apparatus can be operated with or without heating between the first and second vessels. The use of a third vacuum vessel is provisionally claimed. The main intent of the apparatus in the McCurdy patent is to allow condensation of all removed vapor by means of normal cooling water rather than by means of refrigerated water, thereby saving operational costs.
Ando et al., in U.S. Pat. No. 4,537,954 (Aug. 27, 1985), disclose a three-stage devolatilization process for removing volatile components. Each stage is specified as consisting of a vertical foaming preheater and one vacuum vessel. The third stage is operated at a pressure of 50 Torr or less in the presence of a highly volatile stripping agent, such as steam.
All of the above process and apparatus systems have drawbacks and limitations. In some cases, the limitations relate to the degree of devolatilization that can be accomplished. In other cases, the limitations relate to the kinds of liquid that can be effectively devolatilized with a specific system or piece of equipment. In each case, two or more separate vessels are required to serve as the vacuum devolatilizers, which leads to an expensive and bulky apparatus.
These and other difficulties experienced with the prior art systems have been obviated in a novel manner by the present invention.
The applicants conducted extensive research and investigations, including computer modeling and testing various sizes of the apparatus, towards improving the apparatus employed in a continuous unit operation, by the use of a single vessel to achieve equal or better devolatilization results than systems using two separate vessels.
It is, therefore, an outstanding object of the present invention to provide apparatus and methods that increase the degree of devolatilization of polymers achieved by the system.
Another object of this invention is to provide an apparatus and method that reduce the equipment space required to effectively devolatilize a liquid stream.
Another object of this invention is to provide an apparatus that is less expensive to build than previous systems.
Another object of the present invention is to provide apparatus and methods that increase the range of kinds and physical properties of liquid streams that the system can effectively devolatilize.
Another object of the invention is to provide an apparatus and method for retrofitting existing equipment or systems to increase the removal of volatile compounds from polymers.
Another object of the invention to provide a devolatilization system that is less expensive to operate and maintain than previous systems.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.